build: Add script to generate extra images

Change-Id: I53081c7a2b4f18bc8c8a27b04d311a6b078b2022
Use CM standard qcom naming

Change-Id: I360b3dcf1fe82140a7993189e881f9d77b571bef
build: Include pre-pathmap extra config

 * We need to mess with the pathmap for building simplerecovery in user
   builds, but previously there was no way to do this externally.
 * Source an optional config file before loading the pathmap so that we
   can do some magic without having to set environment variables outside
   the normal build system.

Change-Id: I6132f9e6ffa04678378a5a00ba37b98cd4d9f773